The present invention relates to a configuration for converting at least one exhaust emission component of an exhaust emission flow from an internal combustion engine and to a method for the operation of such a configuration.
Various concepts have been developed in order to reduce the environmental pollution from exhaust emissions of internal combustion engines, especially from internal combustion engines used in motor vehicles. These concepts are also aimed at meeting ever lower limits for pollutant emissions.
A problem in the conversion of at least one exhaust emission component of an exhaust emission flow is the period in which the elements of the exhaust emission control system have not yet reached their operating temperature. This is essentially the case during the so-called cold-start phase of the internal combustion engine. During the cold-start phase in internal combustion engines the exhaust emission contains increased quantities of hydrocarbons, since an internal combustion engine usually runs with an excess supply of fuel in the cold-start phase. In addition to unburned hydrocarbons the exhaust emission also contains carbon monoxide. The object during the cold-start phase is to ensure oxidation of the unburned hydrocarbons and the carbon monoxide.
Following the transition from the cold-start phase to a load range of the internal combustion engine the exhaust emissions are both oxidized and reduced, as is the case, for example, with the nitrogen oxides (Nox).
The use of a catalytically active device for the load range, generally referred to as a three-way catalytic converter, is already known. Although the three-way catalytic converter functions excellently in the load range of the internal combustion engine, it cannot convert the unburned hydrocarbons and the carbon monoxide occurring during the cold-start phase sufficiently, so that increased emissions of hydrocarbons and carbon monoxide occur during the cold-start phase. Published, European Patent Application EP 0 485 179 A, for example, discloses an exhaust emission control system containing numerous elements, which might include, for example, a three-way catalytic converter, a hydrocarbon adsorber and a heatable honeycomb element. The configuration of individual elements may vary.
International Patent Disclosure WO 94/17290 further discloses an electrically heated catalytic converter, which at least in some partial areas has a catalytically active coating. The converter, viewed in the direction of flow, has successive electrically heatable areas. The International Patent Disclosure WO 94/17290 is based on the assumption that significant catalytic conversion of the exhaust emission only takes place at temperatures in the order of 300 to 450xc2x0 C.
Both Published, Non-Prosecuted German Patent Application DE 23 33 092 A1 and U.S. Pat. No. 5,417,062 further disclose the configuration of an electrically heatable honeycomb element between two catalytically active honeycomb elements, the intention being to achieve more rapid heating of the catalytically active elements.
Proceeding from this state of the art, the object of the present invention is to specify a configuration for converting at least one exhaust emission component of an exhaust emission flow from an internal combustion engine, by which a reduction in the level of pollutant components, especially of hydrocarbons, is achieved during a cold-start phase. A further object of the invention is to specify a method for the operation of the configuration, by which the configuration achieves a high efficiency.
It is accordingly an object of the invention to provide a configuration for converting at least one exhaust emission component of an exhaust emission flow from an internal combustion engine and a method of operating the configuration that overcome the disadvantages of the prior art devices and methods of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a configuration for converting at least one exhaust emission component of an exhaust emission from an internal combustion engine. The configuration contains a catalytically active first device and a second device disposed downstream of the first device in a direction of a flow of the exhaust emission. The second device includes at least one electrically heatable first honeycomb element, a second honeycomb element having at least one coating being hydrocarbon-adsorbent and disposed downstream of the first honeycomb element, and a electrically heatable third honeycomb element disposed downstream of the second honeycomb element in the direction of the flow of the exhaust emission. A pipe section connecting the first device to the second device is provided, and at least one secondary air line opens into the pipe section.
The first catalytically active device is, for example, a three-way catalytic converter, which forms a so-called main catalytic converter. The main catalytic converter or the first catalytically active device essentially serves for the catalytic conversion of the exhaust emission component of the exhaust emission from the internal combustion engine when the internal combustion engine is in the load range. During the cold-start phase the first catalytically active device makes only a very small, if any, contribution to the conversion of the exhaust emission component of the exhaust emission flow, since the first device has not yet reached the temperature necessary for the catalytic conversion of the exhaust emission component.
The second device, which follows the first device, has at least one electrically heatable first honeycomb element, a second honeycomb element, which has at least one coating that is at least partially at least hydrocarbon-adsorbent, and a third, electrically heatable honeycomb element disposed successively in the direction of flow of the exhaust emission.
By this configuration of the honeycomb elements with their differing functions a reduction in the emission of pollutants, especially of hydrocarbons, is achieved during the cold-start phase.
Owing to the fact that the second honeycomb element has a hydrocarbon-adsorbent coating, the hydrocarbons contained in the exhaust emission are at least partially adsorbed, thereby reducing the emission of hydrocarbons. Disposing a third, electrically heatable honeycomb element downstream of the hydrocarbon-adsorbent honeycomb element ensures that the hydrocarbons not adsorbed by the second honeycomb element and/or the already desorbed hydrocarbons are completely burned. This complete combustion is achieved, in particular, if on the third honeycomb element only the desorbed hydrocarbons pass to the third honeycomb element. This is the case, since the desorption of hydrocarbons is a relatively slow process and the third, electrically heatable honeycomb element reaches its operating temperature very rapidly.
Because a first, electrically heatable honeycomb element is disposed upstream of the second honeycomb element having the hydrocarbon-adsorbent coating, this ensures that the second honeycomb element is heated up relatively quickly. Given a very rapid temperature increase in the second honeycomb element, the hydrocarbon resorption rate is then the defining variable for the catalytic conversion of the unburned hydrocarbons, since desorption is a slower process than the catalytic conversion of the hydrocarbons.
In order to achieve the lowest possible emission of hydrocarbons during the cold-start phase it is also proposed that the first honeycomb element have a coating that adsorbs hydrocarbons. Greater hydrocarbon adsorption is thereby achieved.
According to a further advantageous development of the configuration it is proposed that the first honeycomb element have a catalytically active coating that promotes oxidation. In view of the fact that the first honeycomb element is electrically heatable and the operating temperature is attained relatively quickly, a reduction of carbon monoxide is also achieved by the catalytically active coating.
According to this advantageous development of the configuration it is also proposed that the second honeycomb element have a catalytically active coating that promotes oxidation.
In view of the fact that the third honeycomb element reaches its operating temperature relatively quickly, i.e. the temperature at which a catalytic conversion of the exhaust emission component of the exhaust emission occurs, it is proposed that the third honeycomb element have a catalytically active coating that promotes oxidation and optionally also reduction.
In order to achieve a temperature increase of at least the second honeycomb element having the hydrocarbon-adsorbent coating as quickly as possible, it is proposed that the honeycomb element have a low heat storage capacity. This ensures that only a small exchange of heat takes place between the exhaust emission and the honeycomb element. Therefore, the exhaust emission experiences only slight cooling, if any, as a result of which only a small supply of heat through the two heatable honeycomb elements is needed in order to achieve a reduction of the pollutant emission. The low heat storage capacity is preferably achieved in that the honeycomb element is formed from a sheet metal having a thickness of 15 to 65 xcexcm, preferably of 30 xcexcm.
In order to facilitate assembly of the configuration it is proposed that the second device form a modular unit.
It is proposed, in particular, that the first honeycomb element and/or the third honeycomb element be connected to the second honeycomb element by suitable, electrically insulating fasteners, so that the element(s) is/are fastened by the second honeycomb element. Such fasteners are disclosed, for example, in Published, Non-Prosecuted German Patent Application DE 44 34 673.5 A1.
In order to achieve oxidation of the exhaust emission component, especially oxidation of unburned hydrocarbons, it is proposed according to a further advantageous development of the invention that at least one secondary air line open into a pipe section connecting the first device and the second device. Secondary air can be introduced into the exhaust emission flow through the secondary air line, so that sufficient oxygen is present for oxidation of the hydrocarbons.
In order to provide sufficient oxygen or air for conversion of the unburned hydrocarbons, it is proposed that an oxygen sensor be disposed between the secondary air line and the second device.
Such an oxygen sensor is also referred to as a xcex-probe (lambda probe). This can determine the oxygen content of the exhaust emission flow entering the second device.
It is correspondingly proposed that the oxygen sensor be connected to a control unit, by which a volumetric flow of the secondary air, fed into the exhaust emission by way of the secondary air line, can be controlled.
Should the internal combustion engine be operated at full load immediately following an idling phase, there is a risk that the catalytic reaction in the area of the second honeycomb element will be interrupted by the large volumetric flow of the exhaust emission, since the exhaust emission flow cools in and downstream of the first device, so that the temperature in the second device falls below the temperature necessary for the catalytic reaction. In order to prevent this it is proposed to lead only part of the exhaust emission flow by way of the second device. In specific terms it is proposed that a bypass line be provided, through which at least a part of the exhaust emission can be led past the second device. Devices for varying the flow cross section of the bypass line are disposed in the bypass line, thereby achieving a specific variation of the volumetric flow in the second device.
The electrically heatable honeycomb elements constitute energy consumers. In order to minimize the energy consumption of the electrically heatable honeycomb elements it is proposed that a temperature measuring device be provided, which measures the temperature of the exhaust emission flow downstream of the second honeycomb element, preferably between the second honeycomb element and the third honeycomb element. It can be determined, as a function of the measured temperature, when the hydrocarbons in the second honeycomb element are desorbed, and the electrical heating of the first and/or the second honeycomb element can thereby be switched off.
In order to further improve the conversion of the exhaust emission component of the exhaust emission it is proposed that at least one secondary air line, through which secondary air can be fed into the exhaust emission flow, open into an exhaust pipe connecting the first device to the internal combustion engine.
In order to determine the volumetric flow of the secondary air it is proposed that an oxygen sensor be disposed in the exhaust pipe upstream of the secondary air line.
The oxygen sensor is preferably connected to a control unit, which controls a mixture formation of the internal combustion engine.
Coatings, especially hydrocarbon adsorbers, may be damaged by high temperatures. Owing to the fact that the functionality of the configuration according to the invention essentially does not vary as a function of the temperature of the exhaust emission, the second device can be disposed relatively remote from the internal combustion engine. This reduces the risk of any damage to the coatings. Therefore, it is proposed in particular that the second device be disposed so remote from an internal combustion engine, viewed in the direction of flow of the exhaust emission, that the temperature inside the second device is less than 700xc2x0 C. in essentially all operating conditions of the internal combustion engine.
According to a further idea of the invention, a method is proposed for converting the exhaust emission component from the internal combustion engine, in which electrical heating of the first and the third honeycomb element occurs, at the latest, on starting of the internal combustion engine. This ensures rapid heating of the two honeycomb elements, especially of the last honeycomb element in the direction of flow of the exhaust emission, thereby also reducing the pollutant emission.
In particular it is proposed that an actual temperature of the exhaust emission flow be determined downstream of the second honeycomb element, preferably between the second honeycomb element and the third honeycomb element, and that the electrical heating of the first and/or third honeycomb element be switched off as a function of a comparison between the actual temperature and a set temperature. The actual temperature downstream of the second honeycomb element in particular provides information on whether all hydrocarbons are desorbed at the second honeycomb element and have been subjected to an exothermic conversion.
According to a further advantageous development of the method it is proposed that the actual temperature of the exhaust emission flow be determined downstream of the second honeycomb element, preferably between the second honeycomb element and the third honeycomb element, and that a volumetric flow of the secondary air to be fed to the second device in dependence on a comparison between the actual temperature and a set temperature. This ensures that there is sufficient oxygen available, which is needed for the conversion of at least one exhaust emission component, especially the hydrocarbons.
As an alternative or in addition to the temperature measurement a stoicheiometric ratio of the exhaust emission is preferably determined essentially at an inlet to the second device. A volumetric flow of the secondary air is fed to the second device as a function of a comparison between the stoicheiometric ratio determined and a default stoicheiometric ratio. It is proposed, in order to determine the stoicheiometric ratio, that this be done on the basis of a measurement by an oxygen sensor.
As already stated, there is a risk that in the transition from an idling phase of the internal combustion engine to a full load range, the catalytic reaction will be interrupted by the large exhaust emission volumetric flow in the second device. In order to prevent this it is proposed that only a part of an exhaust emission be fed to the second device as a function of a speed of the internal combustion engine. The other part is led past the second device by way of a bypass line.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a configuration for converting at least one exhaust emission component of an exhaust emission flow from an internal combustion engine and a method of operating the configuration, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.